In recent years, to respond to the data volume of increasing radio communication, a mobile communication system using an OFDMA (Orthogonal Frequency Division Multiple Access) capable of achieving high frequency utilization efficiency is put to practical use. In a mobile phone system thereof, a standard of an LTE (Long Term Evolution) system is laid down in a 3GPP (3rd generation partnership project). Also, a standard of an LTE-A (LTE-Advanced) system obtained by further evolving the LTE system is being processed.
In the LTE and LTE-A systems, in addition to an OFDMA, a MIMO (Multi-Input Multi-Output) technology for performing radio communication by using a plurality of transmitting and receiving antennas is used in order to further improve frequency utilization efficiency.
In the radio communication using the MIMO technology, different signals are simultaneously transmitted by a plurality of antennas on the transmission side, and their signals are received by a plurality of antennas on the reception side. In thus received signals, the signals transmitted by the plurality of antennas are mixed and demodulation processing for separating signals in each transmitting antenna from the received signals is performed on the reception side.
With regard to a demodulation technique of MIMO, various techniques are used. Examples of the typical technique include MLD (Maximum Likelihood Detection), MMSE (Minimum Mean Square Error), and ZF (Zero Forcing). In the MLD, an error rate may be minimized; however, the calculation load is large. On the other hand, the MMSE and ZF each have error rates poorer than that of the MLD; however, the calculation load smaller than that of the MLD because they are linear processing.
In terms of the number of radio terminals (users) on the reception side, a MIMO system is classified into two, namely, a SU-MIMO (Single User-MIMO) mode and a MU-MIMO (Multi User-MIMO) mode. In the SU-MIMO mode, signals to one radio terminal are transmitted by a plurality of antennas on the transmission side. On the other hand, in the MU-MIMO mode, signals to a plurality of radio terminals are simultaneously transmitted by different antennas on the transmission side. In DL (Downlink) communication of the LTE system, 2×2 MU-MIMO modes using UE-Specific RS are regulated as a Transmission Mode 8. In the LTE-A system, MIMO systems up to 8×8 are further regulated as a Transmission Mode 9. Since the possibility that the MU-MIMO mode is used in the LTE-A system is also left, it is considered that importance of the MU-MIMO mode more increases in the future.
In the MU-MIMO mode, even if a modulation scheme of the other multiplexed user is not known, a radio terminal demodulates a received signal by using the MMSE and ZF. On the other hand, in the MLD, a radio terminal needs to know a modulation scheme applied to its own and a modulation scheme of the other multiplexed user in order to demodulate a received signal. The reason is that in the MLD, the modulation scheme applied to its own and the modulation scheme of the other multiplexed user are needed in order to generate replicas of the transmitted signals.
Suppose, for example, that the modulation scheme applied to the transmitted signals of its own is QPSK (Quadrature Phase Shift Keying). Suppose, further, that the modulation scheme applied to the transmitted signals of the other multiplexed user is 16 QAM (Quadrature Amplitude Modulation). When knowing the modulation schemes of QPSK and 16 QAM, the radio terminal generates 4×16 replicas and performs MLD.
Conventionally, there is proposed a receiving unit, control program, and reception control method in a MU-MIMO system in which a calculation load based on a QRM-MLD technique for signal separation is reduced (see, for example, Japanese Laid-open Patent Publication No. 2008-228145).
Further, there is proposed a communication device and a communication method, wherein maximum likelihood detection is applied for reception-side signal detection algorithm in a MU-MIMO system (see, for example, Japanese Laid-open Patent Publication No. 2008-118380).
In some mobile communication systems, a modulation scheme applied to a transmitted signal of its own is notified from the transmission side; however, a modulation scheme applied to a transmitted signal of the other multiplexed radio terminal is not notified from the transmission side. In this case, there arises a problem that in the MLD, a radio terminal fails to demodulate the received signal.